Just a thought - I might be wrong - but it could also be dependent on the certification status of the aircraft. Some older aircraft may still be flying on a full C of A instead of a permit to fly (CAA) or registered as Experimental (FAA). Say a B25 was once operated commercially on a full C of A, wouldn't the pilot in those days have had a type rating instead of a LOA? Ergo, isn't a LOA only issued for the 'Experimental' warbirds?
Sad news about the He111, may they rest in peace.